Methods of molding non-woven carbon fiber mats and related molded products

ABSTRACT

Methods for making a molded non-woven carbon fiber containing mat are provided, along with the resulting mats. Furthermore, methods are generally provided for using the molded non-woven carbon fiber containing mat to form a molded product, along with the resulting molded products.

PRIORITY INFORMATION

The present application claims priority to U.S. Provisional PatentApplication Ser. No. 61/784,049 titled “Methods of Molding Non-WovenCarbon Fiber Mats and Related Molded Products” of Ganesh Deka filed onMar. 14, 2013 and to U.S. Provisional Patent Application Ser. No.61/874,432 titled “Methods of Molding Non-Woven Carbon Fiber Mats andRelated Molded Products” of Ganesh Deka filed on Sep. 6, 2013, thedisclosures of which are incorporated by reference herein.

BACKGROUND

Carbon fibers are widely used as reinforcements in composite materialstructural products. Most typically carbon fibers are combined withpolymeric matrix materials through a variety of manufacturing processesand molded into structural parts used in a wide variety of applications,including airplanes, sporting goods, boats, automobiles and numerousother consumer and industrial products.

Often the carbon fibers are used to produce intermediate products suchas prepregs, fabrics, molding compounds, and other product formssupplied to molders for further processing into structural components.Each of these intermediate product forms has application limitationswith positive and negative attributes, with added cost to supply chainbeing the most negative.

Further, carbon fibers are typically produced as very large filamentcount tows, typically 3,000 to over 50,000 carbon filaments in a towbundle. The carbon filaments have a diameter of approximately 7.2microns (10⁻⁶ millimeter) or less. The fibers and tows may be, forexample, manufactured as described in U.S. Pat. No. 6,385,828 of Kiss,et al., which is incorporated herein by reference. This combination oflarge filament count and small filament diameter gives a very high totalsurface area within the carbon fiber tow which makes fully wetting thesefilaments with a polymeric matrix material very difficult. This completewetting of the filaments facilitates achieving good structuralproperties in the composite material.

In addition, the molding step requires that this matrix flows to fillthe mold cavity, and the rate at which this flow takes place is animportant factor in the efficiency of the molding process, along withthe resulting product.

When the fibers become longer (e.g., over 1 inch) and/or a larger numberof fiber filaments group together, it becomes increasingly difficult todisperse the fibers, creating very real processing issues. Inparticular, the failure to uniformly disperse such filaments inhibitsthe flow properties of the product, which can lead to the creation ofvoid spaces and other anomalies in the final product.

SUMMARY

Objects and advantages of the invention will be set forth in part in thefollowing description, or may be obvious from the description, or may belearned through practice of the invention.

Methods are generally provided, in one embodiment, for making a moldednon-woven carbon fiber containing mat, along with the resulting mats.Furthermore, methods are generally provided for using the moldednon-woven carbon fiber containing mat to form a molded product, alongwith the resulting molded products.

Other features and aspects of the present invention are discussed ingreater detail below.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a photograph of a SMC composite made with the non-woven carbonfiber mat containing bi-component binder fibers in accordance with theinvention.

FIG. 2 is a photograph of a SMC composite made with a comparativenon-woven carbon fiber mat containing 3.7% PVOH binder fibers.

FIG. 3 is a photograph of a SMC composite made with a comparativenon-woven carbon fiber mat containing 7.2% PVOH binder fibers.

FIG. 4 is a photograph of a SMC composite made using a spiral flow moldwith the non-woven carbon fiber mat containing bi-component binderfibers in accordance with the invention.

FIG. 5 is a photograph of a SMC composite made using a spiral flow moldwith a comparative non-woven carbon fiber mat containing 3.7% PVOHbinder fibers.

FIG. 6 is a photograph of a SMC composite made using a spiral flow moldwith a comparative non-woven carbon fiber mat containing 7.2% PVOHbinder fibers.

FIGS. 7A and 7B are photographs of SMC composites made using a spiralflow mold, 7A containing an unfilled resin system and 7B containing afilled resin system.

FIGS. 8A and 8B are photographs showing the spiral flow characterizationof mats formed with 75 and 300 gsm substrates, respectively.

FIGS. 9A, 9B, and 9C show the flow characterization of mats formed with75 gsm substrates.

FIGS. 10A and 10B show the flow characterization of mats formed with 300gsm substrates.

DETAILED DESCRIPTION

Reference now will be made to the embodiments of the invention, one ormore examples of which are set forth below. Each example is provided byway of an explanation of the invention, not as a limitation of theinvention. In fact, it will be apparent to those skilled in the art thatvarious modifications and variations can be made in the inventionwithout departing from the scope of spirit of the invention. Forinstance, features illustrated or described as one embodiment can beused on another embodiment to yield still a further embodiment. Thus, itis intended that the present invention cover such modifications andvariations as come within the scope of the appended claims and theirequivalents. It is to be understood by one of ordinary skill in the artthat the present discussion is a description of exemplary embodimentsonly, and is not intended as limiting the broader aspects of the presentinvention, which broader aspects are embodied exemplary constructions.

Methods of molding nonwoven carbon fiber mats are generally provided,along with the resulting molded products. The molded carbon fiber mat isparticularly useful as a reinforcement material in sheet moldingcompounds. In one embodiment, a wet-laid, non-woven mat material isprovided that generally includes a plurality of carbon reinforcementfibers and a plurality of binder fibers. The molded carbon fiber matgenerally has an open, substantially uniform network of the carbonreinforcement fibers stabilized by binder fibers within the mat. Assuch, the resulting carbon fiber mat provides excellent mold flowproperties for downstream molding processes.

The product formed can be a thermoset nonwoven carbon fiber mat or athermoplastic nonwoven carbon fiber mat, depending on the components andmethod utilized to make the mat. Both embodiments (i.e., the thermosetnonwoven carbon fiber mat and the thermoplastic nonwoven carbon fibermat) are discussed in greater detail below.

I. Thermoset Nonwoven Carbon Fiber Mat

In one embodiment, a method of making a thermoset nonwoven carbon fibermolded mat is generally provided. The method includes forming (e.g., viamixing) an aqueous slurry with a combination of carbon fibers and binderfibers in water. For example, the aqueous slurry can include, in certainembodiments, about 0.2% to about 10% by weight (based on the weight ofthe slurry of a plurality) of carbon fibers and about 1% to about 10% byweight (based on the weight of the slurry) of a plurality of binderfibers.

Additional components may also be included in the aqueous slurry, asdesired. For example, a viscosity modifier can be added to the waterforming said aqueous slurry, either before, concurrent with, or afterthe other components are added to form the slurry. In one embodiment, atwo component system of surfactant and flocculant may be used in saidviscosity modification step of the slurry.

A. Carbon Fibers

Carbon fibers that may be used include fibers made entirely from carbonor fibers containing carbon in amounts sufficient so that the fibersprovide desired structural properties such as tensile strength, modulusand flexural strength. For example, carbon fibers may be used that areformed from a polyacrylonitrile polymer. In particular embodiments, thecarbon fibers may be formed by heating, oxidizing, and carbonizingpolyacrylonitrile polymer fibers. Such fibers typically have high purityand contain relatively high molecular weight molecules. For example, thefibers can contain carbon in an amount greater than about 85% by weight(e.g., about 85% to 100%), such as greater than about 88% by weight,such as greater than about 90% by weight, such as in an amount greaterthan 92% by weight, such as in an amount greater than about 95% byweight.

Of particular advantage, the formed carbon fibers can be chopped to anysuitable length. In one aspect, for example, chopped carbon fibers maybe incorporated into the aqueous slurry having a length of from about 1mm to about 37 mm, such as from about 3 mm to about 6 mm. Such fiberscan have an average diameter of from about 3 microns to about 15microns, such as from about 5 microns to about 10 microns. In oneparticular embodiment, the chopped carbon fibers may be incorporatedinto the aqueous slurry having a length of from about 15 mm to about 37mm, such as about 20 mm to about 35 mm. For example, the carbon fibersmay have a length of about 25 mm and an average diameter of about 7microns. Carbon fibers are described in detail in, for example, in U.S.Pat. No. 6,385,828 and U.S. Pat. No. 4,861,653, which are incorporatedherein by reference in their entirety.

The carbon fibers can be virgin fibers or recycled fibers. In themarket, there are a several different types of recycled fibers. However,most of these recycled fibers are coming from materials left in spoolsor waste from production (clippings). However, most of the materials arefrom pyrolysis or solvolysis.

B. Binder Fibers

Due to the non-bonding characteristics of carbon fibers, some form ofbinder material is included to inter-connect or provide network supportfor carbon fibers in forming the dried mat product. The selection of abinder is important in making a mat with good mold flow properties. Thesubstrate must have enough structural integrity to hold together duringweb forming, handling and processing prior to being resin impregnated.However, once combined with a resin (such as a sheet molding compound),a loose fiber network is desirable to ensure flow during the moldingprocess. When impregnated with the resin, such binders should also allowfiber network bond sites to collapse or decouple allowing fibers to flowas a network in the molding process.

In one particular embodiment, the binder fibers are generallynon-adhesive in that permanent bonding does not occur. Instead, thecarbon fibers are held by melt bonds that are subject to release andreforming during the molding step, thereby resulting in improved moldflow. The binder fibers can also be flexible under mat formingconditions and entrap the carbon fibers without locking them in place bypermanent, i.e., non-melting, adhesive bonding. This feature permitsrapid flow of the matrix under conditions of molding. The term“non-adhesive” means, as used herein, that the multi-component binderfibers are capable of forming melt attachments to carbon fibers that canbe released in the molding step(s) by re-melting and thereby improvemelt flow and subsequently reform after the molding step(s) and in thefinal molded product. It has been discovered that the use ofnon-adhesive, multi-component binder fibers provide enhancedinter-connection of the carbon fibers and faster flow properties in themolding process. In one particular embodiment, the binder fibers includethermoplastic multi-component binder fibers (e.g., bi-component binderfibers).

In one embodiment, the binder fibers are multi-component binder fibers,such as in the form of bicomponent polymer fibers, tricomponent polymerfibers, or other variations. The bicomponent fibers may be arranged in asheath-core, side-by-side, islands-in-the-sea, or segmented-piearrangement. In one particular embodiment, the bicomponent fibers areformed in a sheath-core arrangement in which the sheath is formed of afirst polymer that substantially surround a core formed of a secondpolymer. That is, the sheath is constructed from a first polymericmaterial, and the core is constructed from a second polymeric material,with the first polymeric material being different than the secondpolymeric material. In bicomponent polymer fiber with a sheath-corearrangement, it is not required that the sheath totally surround thecore.

In a particular embodiment, the first polymeric material (i.e., thesurface component) has a melting point lower than the melting point ofthe second polymeric material (i.e., the core component) so that uponheating the bicomponent fibers to a temperature above the melting pointof the first polymeric material and below the melting point of thesecond polymeric material, the first polymeric material will soften ormelt while the second polymeric material remains intact. In oneembodiment, the surface component (e.g., the first polymeric materialthat forms the sheath) has a melting point within the range of the matprocessing conditions to allow it to soften and flow at the matprocessing conditions. However, the core component (e.g., the secondpolymeric material that forms the core) has a melting point above therange of the mat processing conditions to allow it to remainsubstantially intact at the mat processing conditions.

Numerous combinations of materials can be used to make the bicomponentpolymer fibers, such as, but not limited to, combinations usingpolyester, polypropylene, polysulfide, polyolefin, and polyethylenefibers. Specific polymer combinations for the bicomponent fibers includepolyethylene terephthalate/polypropylene, polyethyleneterephthalate/polyethylene, and polypropylene/polyethylene. Othernon-limiting bicomponent fiber examples include copolyester polyethyleneterephthalate/polyethylene terephthalate (coPET/PET), poly 1,4cyclohexanedimethyl terephthalate/polypropylene (PCT/PP), high densitypolyethylene/polyethylene terephthalate (HDPE/PET), high densitypolyethylene/polypropylene (HDPE/PP), linear low densitypolyethylene/polyethylene terephthalate (LLDPE/PET), nylon 6/nylon 6,6(PA6/PA6,6), and glycol modified polyethylene terephthalate/polyethyleneterephthalate (6PETg/PET).

The bicomponent polymer fibers may have a denier of from about 1-18denier and a length of from 2-12 mm. In one embodiment, the firstpolymer (surface component) has a melting point within the range of fromabout 150 to 400° F., or in the range of from about 170 to 300° F. Thesecond polymer (core component) has a higher melting point, for example,above about 350° F.

The surface component(s) of the binder fibers are selected so as tosoften and lend flexibility to entrap carbon fibers but not to theextent that permanent adhesive attachment occurs and the permanentlyattached carbon fibers are locked in place by adhesive bonds under themat forming conditions.

Multi-component fibers are available in many different combinations andmay be selected with the above parameters in mind. Additionalcharacteristics include fiber length and diameter. Generally, usefulmulti-component fibers have lengths broadly within the range of fromabout 3 mm to about 20 mm, for some embodiments from about 6 mm to about12 mm and diameters within the range from about 1.5 dpf to about 4 dpf,for some embodiments within the range of from about 2 dpf to about 3dpf.

In one embodiment, the binder fibers may be present in the aqueoussuspension in an amount of at least about 2% by weight based upon theweight of all fibers present and up to about 10%, for example within therange of from about 2% to about 4%.

C. Wet-Laid Mat

The slurry can be deposited onto a porous forming surface to form asupported wet-laid mat. Water can be then removed (e.g., drained) fromthe wet-laid mat through the porous forming surface.

The open network within the mat is formed during the wet laid process ofthe aqueous suspension/slurry. As the wet mat is drained through theporous forming surface, it may form a gradient structure because ofdraining from one direction and compacting as the mat is formed. In oneembodiment, a size coating may be applied by curtain or other coatingmeans to improve wet-out of fibrous mat, and therefore reduce voidvolume in the final product.

In one embodiment, the plurality of carbon fibers are substantiallyaligned after the wet laid process, due to the typical fiber alignmentof cellulose or synthetic fibers in a wet laid web and have more fibersoriented in one direction than in any other directions.

The liquid (e.g., water and any other solvents present) can be removedvia suction-assisted draining through the porous forming surface, viapressing, etc.

In one embodiment, the method further includes a step of applying asizing agent coating fibers of said wet-laid mat prior to activatingsaid multi-component binder fibers. Manipulation of the wet-laid processcan determine the ultimate degree of dispersion and thus placement ofthe fiber as an individual filament or as a tow bundle. In order tocontrol fiber placement and entanglement, the dispersion of the clusterof fiber tows can be manipulated by adding a dispersing aide such as aflocculant. A non-limiting example of a suitable flocculant is Nalclear7768 ethoxylated alcohol from Nalco. Further enhancement may be obtainedby also adding a surfactant. A non-limiting example of a suitablesurfactant is Nalco 8493. The flocculant and the surfactant may each beused in amounts commonly associated with wet-laid processes. In oneembodiment, the flocculant and the surfactant may each be added in anamount within the range of about 0.02% to about 0.04%, or about 0.005%to about 0.006%, based on the total weight of the suspension. Withoutthese additives, the carbon fibers have a greater tendency to be clumpedtogether as bundles.

In one embodiment, a dispersant may first be added to the white (slurry)water at about 600-800 ppm with substantial mixing energy. An example ofa suitable dispersant is Nalco 8493 from Nalco Company, Naperville, Ill.A high molecular weight anionic flocculant, such as Nalclear 7768 fromNalco Company, may then be added to maintain the white water viscosityat about 5 to about 10 cps Brookfield at 21° C. measured with #2 spindleat 30 rpm.

By adding to the water a dispersant, flocculant and/or surfactantadditives and mixing of the carbon fibers under very low agitation,dispersed fibers with minimal clumping can be obtained. The dilutionwater used for carbon fiber supplied to the forming unit also may beadjusted in the same manner, with the provision that increased viscositymay at some addition level adversely impact the drainage characteristicsof the mat to be formed.

Other additives may be included in the aqueous suspension provided suchadditives in the chosen amounts are not detrimental to the carbon fibermatrix. Non-limiting examples of such additives include additionaldispersants, surfactants, antioxidants, anti-foamers, bactericides,oxidation absorbers, thickeners, softeners, hardners, UV stabilizers,and colorants.

The ability to control the fiber orientation allows for the specifictailoring of the mechanical properties of the resulting mat.Specifically, control of fiber orientation allows the mat to be tailoredto the application and to the specific process.

D. Activating the Binder Fibers

After wet laying the aqueous slurry and removing at least a portion ofthe liquid, the binder fibers are activated to form a melt attachmentbetween the surface of the binder fibers and the carbon fibers in thewet-laid mat. The binding fibers can then be activated by heating to atemperature at or above the melt softening temperature point whereby thebinder fibers form a melt attachment to the carbon fibers at contactpoints between the carbon fibers and the binder fibers.

For example, when the composition of the bicomponent sheath-core binderfiber discussed above, the surface component (e.g., the first polymericmaterial of the sheath) is melted to melt bond to the carbon fibers.While the heat applied is sufficient to melt the first polymericmaterial of the sheath (e.g., at or above the melt temperature of thefirst polymeric material), the heating temperature is less than themelting point of the second polymeric material of the core to ensurethat the binder fibers substantially retain their integrity. In oneembodiment, infra-red heating can be utilized to activate the binderfibers. For example, when composed of a multi-component fiber, thebinder fibers can be heated to a temperature generally at or above themelting point of the surface component to form melt bonds with thecarbon fibers.

To initially melt the surface component(s) of the non-adhesivemulti-component binder fibers, the mat may be placed on the surface of arotating heated Yankee dryer and dried. Alternatively, the mat may beplaced on a through air dryer with hot air passing through the mat. Inaccordance with a preferred embodiment of the present disclosure, themat is dried on a flat wire with hot air being passed through the mat.In a further aspect, drying may be a two-step process, for example usinginfra-red heating immediately after mat formation so as to initiallymelt the surface component(s) of the multi-component binder fibers priorto final drying. For example, leaving the porous draining surface, watercontent may be reduced to less than 30% and less than about 2% afterfinal drying.

The open uniform network morphology provides for very easy impregnationof the mat with polymeric or other composite matrix materials and easywetting of individual filaments. This in turn leads to excellentmechanical and other physical properties of the composite with very highfiber property translation into the composite structure. The ease ofimpregnation leads to higher speed/lower cost downstream processing,thus leading to lower cost structural components. The open networkmorphology also allows impregnation with powder matrix materials andhigh viscosity liquid matrix materials that are typically difficult orimpossible to achieve with other carbon fiber products and processes.

The open network morphology of the mat product and unique processingallow for several distinguishing features. One feature is the ability tocontrol the mat physical and mechanical properties. The unique abilityto control the fiber orientation allows for the specific tailoring ofthe mechanical properties of the mat. Specifically, it allows the mat tobe tailored to the application and to the specific process. Anotherfeature is the open uniform fiber network morphology.

Further drying the wet-laid mat forms a non-woven carbon fiber mathaving an open uniform network of carbon fibers maintained insubstantial alignment by the activated binder fibers. That is, furtherdrying the wet-laid mat forms a non-woven carbon fiber mat having anopen uniform network of carbon fibers substantially maintained inposition by the melt bonding multi-component fibers without permanentadhesive bonding between the carbon fibers and multi-component fibers.At this stage, the carbon fibers are entrapped in the mat, but notlocked in place by any permanent adhesive bonds.

E. Impregnated, Non-Woven Carbon Fiber Mat

A carbon fiber reinforced composite can then be made by preparing anon-woven carbon fiber mat (as discussed above, including carbon fibersand non-adhesive, multi-component binder fibers); impregnating thenon-woven carbon fiber mat with a resin; and molding the resinimpregnated non-woven carbon fiber mat to form a carbon fiber reinforcedcomposite. In this embodiment, the multi-component fibers may beselected with a surface component that is compatible with theimpregnating resin to further facilitate the molding step. For example,molding can be achieved by flowing the non-woven carbon fiber mat into amold cavity (e.g., completely filling a mold cavity). As such, theimpregnated mat can be subjected to vacuum assisted resin transferconditions to cause mold flow and form a molded product; followed bysubjecting the dried wet-laid mat to temperature, pressure and/or vacuumassisted resin transfer conditions re-melting at least the binder fibersurface component(s) to form a molded product.

The open and uniform network morphology of substrates provides very easyimpregnation with polymeric or other composite matrix materials and easywetting of individual filaments particularly where the multi-componentbinder fibers have a surface component that is compatible with theimpregnating resin. By compatible it is meant that with the curingsystem employed for use with a thermosetting resin matrix, the surfacecomponent(s) of the multi-component binder fibers, co-cure with thematrix forming bonds that result in enhancing physical and otherproperties. In many embodiments, the resin and surface component(s) havesimilar chemical structures and/or properties and result in little or noobservable remaining binder particles as contaminant after the moldingstep. An example includes a CoPET/PET bicomponent binder fiber andunsaturated polyester impregnating resin. Another example is the use ofa vinyl ester or polyester in combination with similar vinyl ester orpolyester containing multi-component binder fibers.

The impregnating resin may be selected from a wide variety ofcompositions depending on the desired properties of the molded productas will be apparent to those skilled in the art of molding suchproducts. For molded products of the type described above, frequentlythe resin will be selected from thermoset resins such as epoxy and vinylester compounds, for example, in addition to the unsaturated polyesterresins mentioned above. The amount of saturant used will also bedetermined by the properties desired in the molded product but may, forexample, be within the range of from about 30% to about 55% or incertain embodiments from about 40% to about 50%, based on the volume ofthe impregnated mat.

The open network morphology and ease of impregnation proves very usefulfor downstream applications such as SMC compounding and compressionmolding. Additional features of the carbon fiber mat include control ofthe surface finish of the final product, and efficient fiber translationinto composite structures.

In one embodiment, multiple layers of the impregnated, wet-laid mats canbe combined prior to subjecting them to the conditions for molding.

In the resulting mat, the binder fibers can be about 1% to about 5%, byweight, of the final product (e.g., about 2% to about 4% by weight).

II. Thermoplastic Nonwoven Carbon Fiber Mat

In another embodiment, the molded non-woven carbon fiber containing matcan be formed from an aqueous slurry comprising carbon fibers,thermoplastic polymeric fibers, and binder fibers. The carbon fibers andthe binder fibers can be independently selected from those discussedabove. Additional components, as discussed above, can also be includedin the aqueous slurry.

In this embodiment, the binder fibers can be present in an amount ofabout 0.5% to about 5% by weight based on the weight of the slurry; thecarbon fibers can be present in an amount of about 10% to about 90% byweight based on the weight of the slurry; and the thermoplastic fiberscan be present in an amount of about 10% to about 90% by weight based onthe weight of the slurry. Suitable thermoplastic fibers for use in thisembodiment can include nylon, a polyolefin, a polyamide (e.g., PA6,PA66), polyphenylenesulfide (PPS), a polyetheretherketone (PEEK), apolyetherimide (PEI), or mixtures thereof.

This aqueous slurry can then be deposited onto a porous forming surface(as discussed above) to form a wet-laid mat supported by the porousforming surface, and at least a portion of the water can be removed fromthe wet-laid mat through the porous forming surface.

Then, the wet-laid mat can be heated to a flow temperature such that thethermoplastic polymeric fibers melt. This flow temperature is, in oneembodiment, above the melting point of the thermoplastic polymermaterial of the thermoplastic fibers, but below the melting point of thebinder fibers. For example, when formed from bi-component fibers havinga first polymeric material with a first melting temperature forming thesheath and a second polymeric material having a second meltingtemperature forming the core, the flow temperature can be below thefirst melting temperature and/or the second melting temperature. Assuch, at the flow temperature, the thermoplastic fibers generally meltand flow, while the binder fibers (and the carbon fibers) substantiallyretain their integrity.

The wet-laid mat, can then flow, while at the flow temperature, into amold where it can then be subjected to heat and pressure to activate thebinder fibers. After cooling, the wet-laid mat within the mold a moldednon-woven carbon fiber containing mat remains.

In the resulting mat, the binder fibers can be about 1% to about 5%, byweight, of the final product (e.g., about 2% to about 4% by weight).

EXAMPLES Example 1

A carbon fiber mat is made using a wet-laid process. CMC 7H (carboxymethyl cellulose Aqualon 7H) in an amount of 0.1 wt %, based on thetotal weight of the water/CMC mixture, is initially dispersed in warmwater (95° F.-100° F.) to raise the viscosity of water to 40 cps. Carbonfibers [Zoltek carbon fibers/size 25 mm/7.8 microns] in an amount of 97wt % and 3.0% by weight bi-component (amorphous CoPET/PET) fibers T201from Fiber Innovation Technology, Inc., Johnson City, Tenn., 2.2 dtex, 6mm having a 110° C. melting sheath are dispersed in the water containingCMC. The resulting slurry is deposited on a porous surface to form asupported wet-laid mat. The water is drained from the wet-laid throughthe porous surface. The bi-component binder fibers are activated byheating to a temperature of about 110° C. to form melt attachments tothe carbon fibers at the contact points between the carbon fibers andthe binder fibers. The wet-laid mat is saturated with water basedpolyurethane (1% solution, Hydrosize U6-01 aqueous polyurethane solutionfrom Michelman, Cincinnati, Ohio) to facilitate sizing the carbon fibersfor subsequent resin wet-out. The wet-laid mat is further dried to forma non-woven carbon fiber mat having an open uniform network of carbonfibers. The carbon fibers are stabilized in the network by the binderfibers.

Comparative Example 2

A wet-laid carbon fiber mat is prepared substantially in accordance withthe method of Example 1, with the exception that the binder fibers are3.7% by weight of polyvinyl alcohol fiber VPB 105-2, 4 mm withdissolving temperature of 60° C. in water and the binder is activated ata temperature of about 62° C.

Comparative Example 3

A wet-laid carbon fiber mat is prepared substantially in accordance withthe method of Example 1, with the exception that the binder fibers are7.2% by weight of polyvinyl alcohol fiber VPB 105-2, 4 mm withdissolving temperature of 60° C. in water and the binder is activated ata temperature of about 62° C.

Example 4

The carbon fiber mat of Example 1 is resin impregnated into a sheetmolding compound using a standard plaque mold. The mat is resinimpregnated with a filled unsaturated polyester resin system compatiblewith the bi-component binder fibers and molded at 1000 psi at 295° F.for 2 minutes. The resin formulation used is a hybrid unsaturatedpolyester/urethane resin that includes a filler to control viscosity, asfollows:

Ingredient Description Parts (wt.) Dion ®31638¹ Unsaturated polyesterpolyol 75 500-75P² Peroxide initiator 1 PBQ 12.5³ P Benzoquinoneinhibitor 0.15 ACF⁴ Zinc stearate, mold release 4 W-4⁵ Calcium carbonatefiller 50 Dion ®31110¹ Diphenylmethane diisocyanate 22.5 ¹Available fromReichhold, Inc., Durham, NC ²Available from Syrgis PerformanceInitiators, Helena, Arkansas ³Available from Plastics Color Corporation,Calumet City, II ⁴Available from Ferro Corporation, Mayfield Heights, OH⁵Available from J.M. Huber Corporation, Edison, NJ

The carbon fiber mat is wetted out with the above resin system targetinga 35% (i.e. ratio of 65/35 or 1.857) carbon fiber weight orapproximately 30% fiber volume. Sheets are cut 10″ square for wettingout and molding in 12″×12″ with mold coverage of 69.4%.

-   -   Dry weight of carbon fiber mat=182 grams (made with 25 mm        chopped carbon fibers with 3% bi-component T201 PET/CoPET        fibers.

For wetting out, 15% excess resin is used=182×1.857×1.15=388 grams FIG.1 shows the flow characteristics of the carbon fiber mat of Example 1used to produce the SMC compound of Example 4.

Comparative Example 5

The carbon fiber mat of Comparative Example 2 is resin impregnated intoa sheet molding compound using a standard plaque mold substantially inaccordance with the process of Example 4.

Dry weight of carbon fiber mat=213 grams (made with 25 mm chopped carbonfiber with 3.7% PVOH VPB 105-2 4 mm as binder fibers)

For wetting out, 15% excess resin was used=213×1.857×1.15=454 grams FIG.2 shows the flow characteristics of the carbon fiber mat of ComparativeExample 2 used to produce the SMC compound of Comparative Example 5.

Comparative Example 6

The carbon fiber mat of Comparative Example 3 is resin impregnated intoa sheet molding compound using a standard plaque mold substantially inaccordance with the process of Example 4.

Dry weight of carbon fiber mat=197 grams (made with 25 mm chopped carbonfibers with 7.2% PVOH VPB 105-2 4 mm as binder fibers)

For wetting out, 15% excess resin used=197×1.857×1.15=420 grams FIG. 3shows the flow characteristics of the carbon fiber mat of ComparativeExample 3 used to produce the SMC compound of Comparative Example 6.

Observations of Wetting Out and Molding, Examples 1-6:

In the production of sheet molding compounds, it is desirable that thereinforcing carbon fibers contact the polymeric matrix material. Ameasure of this contact is referred to as wetting, which is a measure ofhow well the carbon fibers are encapsulated by the matrix material.

The carbon fiber mat of Comparative Example 2: wet only on surfaces, andwas very dry. The sheets were cross plied at 90° to each other toequalize fiber direction.

The carbon fiber mat of Example 1: dry centers but less than that ofComparative Example 2. However, the mold is filled 100% with anacceptable appearance and clearly shows improved flow using bi-componentbinder. This can be quantified using weight and/or photographicmeasurement. As a further demonstration, wet out samples of Example 1were cut in 10″ round instead of 10″ square and it is observed that evenwith round samples, the mold fills up completely.

The carbon fiber mat of Comparative Example 3: Two corners not filled,one carbon fiber high spot. High resin flow compared to fiber flow.About 95% full.

Comparing the flow characteristics in a standard plaque mold, it cantherefore be indicated that the type of binder used for making a carbonfiber substrate has significant influence on the flow characteristics ofa fiber matrix. It has been shown that the use of non-adhesive,multi-component binder fibers for making a carbon fiber substrateaffects mold flow characteristics of a carbon fiber matrix. Thecompatibility between the multi-component binder fibers and the resinused to impregnate the mat is a significant factor. For example, it hasbeen found that use of EVNPP or HDPE/PP bicomponent binder fibers withthe resin described in the above examples resulted in the bicomponentbinder fibers remaining as contaminants in the mold compound.

Example 7

The carbon fiber mat of Example 1 is resin impregnated into a sheetmolding compound using a spiral flow mold device of the type describedin U.S. Pat. No. 4,875,363, incorporated herein in its entirety(SmarTrac® Model sensor from Signature Control Systems, Inc., Littleton,Colo. 80127). In the spiral flow tool, the mold has a 6″ square loadingarea and a 2″ wide channel that is 48″ long for a relative flowmeasurement. It is a compression mold with shear edges and can moldplaques up to 0.75″ thick. Molding pressure can be varied to controlflow in part. A sample of the carbon fiber mat cut to 5 in.×5 in. isresin impregnated with the filled unsaturated polyester resin systemdescribed above, and molded at 1000 psi at 295° F. for 2 minutes.

FIG. 4 shows the flow characteristics of the carbon fiber mat of Example1 in the spiral flow mold used to produce the SMC compound of Example 7.

Comparative Example 8

The carbon fiber mat of Comparative Example 2 is resin impregnated intoa sheet molding compound using a spiral flow mold device substantiallyin accordance with the process of Example 7. FIG. 5 shows the flowcharacteristics of the carbon fiber mat of Comparative Example 2 in thespiral flow mold used to produce the SMC compound of Comparative Example8.

Comparative Example 9

The carbon fiber mat of Comparative Example 3 is resin impregnated intoa sheet molding compound using a spiral flow mold device substantiallyin accordance with the process of Example 7. FIG. 6 shows the flowcharacteristics of the carbon fiber mat of Comparative Example 3 in thespiral flow mold used to produce the SMC compound of Comparative Example9.

As demonstrated by FIGS. 5 and 6, with the carbon fiber mat containing3.7% PVOH binder, the resin material flowed approximately 21.0 inches,while with the carbon fiber mat containing 7.2% PVOH binder, the resinmaterial flowed 18.0 inches. This indicates a restriction of flowbecause of increased binder level. FIG. 4 represents the flowcharacterization with the bi-component polyester binder, which has thehighest flow

It has been shown that another way to influence flow is by controllingthe viscosity of the resin formulation. Unexpectedly, it was discoveredthat within certain parameters, increasing the viscosity of the resinactually improves flow. While the mechanism is not entirely clear, it isbelieved that higher viscosity resin aids the movement of the carbonfibers.

Example 10

The same resin system described above is used to impregnate the carbonfiber mats. For unfilled system, the calcium carbonate filler (W-4) wasexcluded from the resin formulation. Wetted out samples are cut 4.5 in.square to charge the spiral flow system and in both conditions, theresin charge is 228 grams. Testing is done at 295° F. and at 1000 psi.In the filled system, the charge was found to flow 15.5 inches whereasin unfilled system, the charge flowed 13.5 inches. Viscosity of theresin is measured using a Brookfield Engineering Laboratories,Middleboro, Mass. Model HB, viscometer with T-bar Spindle A at 10 rpm.Viscosity of the filled resin system is 560 cps whereas unfilled systemis 450 cps.

As shown in FIGS. 7A and 7B, the filled system with higher viscosityprovides additional force for pushing the fiber network and therebyincreased flow. In this example, the increase is measured by additional2.0 inches of matrix flow.

It was found that the open and uniform network morphology of the carbonfiber mats provided very easy impregnation with polymeric or othercomposite matrix materials and easy wetting of all individual filaments.While not desiring to be bound by theory, it is believed that the lackof permanent adhesive bonds locking carbon fibers in place combined withincreased flexibility of the multi-component non-adhesive binder fiberscontribute to improved flow of both the carbon fibers and resinimpregnate under molding conditions.

It is generally accepted that surface smoothness is related to the resinformulation and add-on percentage. However, binder levels as well as thechoice of binders to control resin flow properties also can enhancesurface features of the final product.

Sheet Molding Compound (SMC)

The carbon fiber mat of the present invention including the open anduniform network morphology allows for very easy impregnation withpolymeric or other composite matrix materials and easy wetting of allindividual filaments. The material has applications in automotive,aerospace, and other industrial or commercial applications in which alow cost, high strength material is desired.

Compression Molding with Thermoplastics

The carbon fiber mat of the present invention combined with interwoventhermoplastic fibers, interleaved sheet stock, or impregnated resinproduces a part with uniform wetting out of the individual filaments anda fully impregnated substrate. The open morphology of the mat providesan open pathway for the high viscosity resin to fully impregnate thelaminate stack producing a consistent carbon/thermoplastic material.

The carbon/thermoplastic mat product has applications in high volume,non-structural applications within the automotive, aerospace, and otherindustrial or commercial applications seeking a higher strength, lowercost replacement to existing thermoplastic materials

Nonwoven substrate mats made in accordance with the present inventioncan include a single homogeneous layer of fibers or may include astratified or layered construction. For example, the wet-laid matconstruction may include two or three layers of fibers. Each layer mayhave a different fiber composition using a divided papermaking headboxas is known. Each of the fiber layers from the divided headbox includesa dilute aqueous suspension of fibers. The particular fibers containedin each layer generally depend upon the construction being formed andthe desired results. In one aspect, for example, the middle layer maycontain different concentrations of fibers in combination with thecarbon fibers while the outer layers, on the other hand, can containmore binder fibers.

Infusion Media—Vacuum Infusion and Resin Transfer Molding (RTM)

The carbon fiber mat of the present invention interleaved or combinedwith standard woven or non-woven materials acting as a structuralinfusion media, significantly increases the rates of infusion andprovides added mechanical benefit over a standard laminate. The timerequired to infuse large scale applications is an important variable tobe considered when evaluating the profitability of a project.

Greatly reducing such times, leads to greater throughput of parts,higher volumes, and lower operating costs. The present mat products haveapplications in large scale VARTM or RTM parts where infusion rates arean important variable in the cost of a project. Such applicationsinclude but are not limited to wind turbine blades, large yachts, andwithin the commercial boating industry.

Example 11 Flowability of Thermoplastic Materials

It appeared there was a significant flow difference of consolidated matsof 75 gsm when compared to 300 gsm, as indicated in FIGS. 8A and 8B. Inorder to ensure repeatability, flowability was repeated in a number ofmats.

Flow characterization of consolidated mats was repeated using 75 and 300gsm substrates with 40% 25 mm carbon fibers+60% 7 dpf 6 mm Polypropylenefibers with 3% PVOH VPB 105-2 4 mm fibers. FIGS. 8A and 8B compare flowsfor these resulting substrates. It takes more time to get to ‘stoplimit’ when molding is done of mats with lower basis weight (75 gsm)compared to higher basis weight (300 gsm). In general, there is a slightincrease in flowability of heavier basis weight mats.

The description of this example is as follows:

Using 40% 25 mm carbon fiber, 60% 7 dpf 6 mm polypropylene fibers and 3%VPB 105-2 4 mm PVOH fibers, handsheets targeting 75 and 300 gsm wereprepared. 25 mm carbon fibers, received from Zoltek in 50K, wereinitially dispersed with a dual polymer surfactant and dispersantsystems. The handsheets were then consolidated in a hot press asfollows:

Sheets were cut 5″ square and stacked to weigh approximately 50 gramseach of 75 gsm and 300 gsm. Using steel plates on top and bottom, thisstack of sheets were pressed in a hot press set at 325° F. under 1000psi loading. It was held in this condition for 30 minutes. Beforeopening the press, temperature was dropped below 150° F.

These consolidated mats were tested for flow characteristics using aSpiral Flow System at Reichhold, RTP, NC. The platens were heated to349° F. Approximately 100 grams of consolidated materials were placed inthe mold and molded at 1000 psi. Molding was done at 1000 psi and heldat molding temperature 348-349° F. for 10 minutes. The mold is cooledbelow 150° F. before opening. The time it took to ‘stop’ was recorded.

As shown in FIGS. 9A-9C and 10A-10B, it appears that only a slightlyhigher flow was observed in 300 gsm consolidated mats. However, therewas significant difference in time to reach to ‘stop’. Whereas it took4.3 minutes to get to ‘stop’ with 300 gsm mats, time to reach ‘stop’ was6.5 minutes in 75 gsm indicating easier flow with heavier mats. As such,there may be a slight advantage in making mats heavier to achieve higherflow in a consolidated mat.

While the invention has been explained in relation to variousembodiments, it is to be understood that various modifications thereofwill be apparent to those skilled in the art upon reading thespecification. The features of the various embodiments of the articlesdescribed herein may be combined within an article. Therefore, it is tobe understood that the invention described herein is intended to coversuch modifications as fall within the scope of the appended claims.

What is claimed:
 1. A method of making a molded non-woven carbon fibercontaining mat, the method comprising: mixing an aqueous slurrycomprising carbon fibers and binder fibers, wherein the aqueous slurrycomprises the binder fibers in an amount of about 1% to about 10% byweight based on the weight of the slurry, and wherein the binder fiberscomprising non-adhesive thermoplastic multi-component fibers; depositingthe slurry onto a porous forming surface to form a wet-laid matsupported by the porous forming surface; removing water from thewet-laid mat through the porous forming surface; activating the binderfibers such that a surface component of the binder fibers forms a meltattachment directly to the carbon fibers; further drying the wet-laidmat to form a non-woven carbon fiber mat having an open network ofcarbon fibers; impregnating the dried, non-woven carbon fiber mat with aresin; and subjecting the impregnated non-woven carbon fiber mat tovacuum assisted resin transfer conditions in a mold to cause mold flowand re-melt the surface component of the binder fiber to form the moldednon-woven carbon fiber containing mat.
 2. The method as in claim whereinthe resin is compatible with the surface component of the binder fibers.3. The method as claim 1, wherein the non-woven carbon mat has an opennetwork of carbon fibers maintained in substantial alignment by theactivated binder fibers.
 4. The method as in claim 1, wherein activatingthe binder fibers comprises: allowing absorption of water by the binderfibers; and thereafter, drying at a temperature of about 60° C. orhigher such that the binder fibers dissolve and work as binder.
 5. Themethod as in claim 1, wherein the non-adhesive thermoplasticmulti-component fibers comprise core-sheath bi-component fibers.
 6. Themethod as in claim 5, wherein the bi-component fibers comprise a firstpolymeric material having a first melting temperature and a secondpolymeric material having a second melting temperature, and wherein thefirst melting temperature is less than the second melting temperature.7. The method as in claim 6, wherein the bi-component fibers are definea sheath and a core, wherein the sheath comprises the first polymericmaterial, and wherein the core comprises the second polymeric material.8. The method as in claim 7, wherein activating the binder fibers suchthat the binder fibers form a melt attachment to the carbon fiberscomprises: heating the binder fibers to a temperature at or above thefirst melting temperature sufficient to melt the first polymericmaterial of the sheath, but less than the second melting temperature ofthe second polymeric material of the core.
 9. The method as in claim 5,wherein the first polymeric material comprises a first polyester, andwherein the second polymeric material comprises a second polyester, thefirst polyester being different than the second polyester.
 10. Themethod as in claim 9, wherein the first polyester has a melt point ofabout 100° C. to about 225° C., and wherein the second polyester has amelt point temperature of about 250° C. or greater.
 11. The method as inclaim 1, wherein the binder fibers comprise core-sheath bi-componentfibers having a first polymeric material having a first meltingtemperature and a second polymeric material having a second meltingtemperature, wherein the sheath comprises the first polymeric material,and wherein the core comprises the second polymeric material, andwherein the resin comprises a polymer compatible with the firstpolymeric material.
 12. The method as in claim 11, wherein the binderfibers comprise CoPET/PET bicomponent fibers, and wherein the resinincludes polyester resin.
 13. The method as in claim 1, wherein theaqueous slurry comprises the carbon fibers in an amount of about 0.2% toabout 10% by weight based on the weight of the slurry of a plurality ofcarbon fibers.
 14. The method as in claim 1, wherein the aqueous slurryfurther comprises a viscosity modifier.
 15. The method as in claim 14,wherein the viscosity modifier comprises a two component system ofsurfactant and flocculant.
 16. The method as in claim 1, whereinactivating the binder fibers includes infra-red heating of the wet-laidmat.
 17. The method as in claim 1, further comprising: combiningmultiple impregnated non-woven carbon fiber mats as layers prior tosubjecting them to the conditions for molding.
 18. The method as inclaim 1, wherein the resin comprises a thermoset resin.
 19. The methodas in claim 18, wherein the thermoset resin comprises an epoxy compound,a vinyl ester compound, or an unsaturated polyester resin.
 20. Themethod as in claim 1, wherein the resin is present within the moldednon-woven carbon fiber containing mat in a range of about 30% to about55% based on the volume of the impregnated mat.